Introduction

Mark Batty recently undertook a partial formalization of the C++
memory model, which Mark summarized in
N2955.
This paper summarizes the discussions on Mark's paper, both verbal and
email, recommending appropriate actions for the Core Working Group.
Library issues are dealt with in a separate N3057 paper.

This paper is based on
N3045,
and has been updated to reflect discussions in the Concurrency subgroup
of the Library Working Group in Pittsburgh.
This paper also carries the C-language side of
N3040,
which was also discussed in the Concurrency subgroup of the Library
Working Group in Pittsburgh.

Qualifying the conflicting accesses as being both non-atomic.
This change is unnecessary, because a single non-atomic operation
in a set of conflicting operations is all that is required to
result in a data race.
This change is further incorrect, because it is possible to
publish a reference to an atomic object within its constructor,
which would permit a data race between an atomic operation from
some other thread on the one hand and the remainder of the
(non-atomic) accesses from the constructor on the other.

Recommendation: no change.

Correct use of simple locks results in simple interleaving, as
currently stated in 1.10p14.
If atomic memory_order_seq_cst
operations are also used outside
of lock-based critical sections, the result is still simple
interleaving.
If atomic memory_order_seq_cst
operations are also used both
inside and outside of lock-based critical sections, the
result is still sequentially consistent, but the individual
lock-based critical sections are no longer simply interleaved.
However, the result will be consistent with at least one simple
interleaving of
the individual operations making up each critical section.

Core Non-Issues

Core Non-Issue 1: 1.10p4 Overhead of Access to Atomic Objects

Atomic and locking objects are not trivially copyable [29.5.1p2, 29.5.2p1,
29.5.3p2], so the result of copying them (for example,
via std::memcpy)
are not specified by the standard [3.9].
Additionally, if the memcpy
operation results in a data race, then undefined behavior is explicitly
specified by the working draft [1.10p14].

There was some spirited discussion of the non-data-race case on the
email reflector, with the following positions outlined:

Peter Dimov argued that atomic integral types have standard layout
[29.5.1p2, 29.5.2p1], and that there was therefore no good reason
to prohibit copying out the underlying memory
locations of an atomic object.
Peter further argued that atomic accesses to large objects
can incur high overheads, even when using
memory_order_relaxed,
and that there are a number of
situations (including some implementations of resizeable
hash tables) where most accesses to a given object are not
subject to data races.
In such cases, there is good reason to avoid
memory_order_relaxed's
overhead for accesses known to be data-race free.

Clark Nelson argued against copying to atomic objects, even in
absence of a data race, given that some implementations might
have non-trivial representations. Clark was willing to
entertain the thought of copying from atomic objects, but only in
absence of data races.

An informal poll of the Core Working Group resulted in the position
that copying non-trivially copyable objects (e.g., via
memcpy)
was at best unspecified, at worst undefined.

Paul McKenney argued that mandating copyability might rule out
active-memory hardware optimizations, and that the behavior
should thus remain undefined.
The effect of copying out an atomic object's
underlying representation can be
efficiently emulated via a memory_order_relaxed load, so
it is not necessary to define the effect of copying the
underlying representation.
Furthermore, the effect of copying an underlying representation
to an atomic object can be both safely and efficiently emulated
via a memory_order_relaxed store for machine-word-sized
accesses, which are the most common in practice.

Some time back, Alexander Terekhov is said to have proposed
an additional memory_orderenum
member that would permit the implementation to access the
atomic object non-atomically (for the purposes of this paper,
call it memory_order_nonatomic).
This could be thought of as specifying memory ordering that is
so relaxed that the implementation need not even guarantee
indivisibility of different accesses to the same
atomic object.
A memory_order_nonatomic
operation would therefore be subject
to data races.

Hans Boehm proposed leaving 1.10p4 as is and stating some
form of the prohibition in clause 29 or 30. Peter Dimov
and Paul McKenney agreed with this approach, with Paul
suggesting 29.3p1.

Therefore, this paper recommends no changes to 1.10p4.
This paper does not recommend adding memory_order_nonatomic
to c++0x, but something similar should be considered for a later TR
or a later version of the standard.

Core Non-Issue 2: 1.10p6 Mathematical Meaning of Maximal

The phrase “M is a maximal contiguous” could
be interpreted as meaning the sequence having the maximum value or
any of a number of alternative interpretations.
However, there were other instances of this abbreviation that were
not objected to, so recommend no change.

Core Non-Issue 3: 1.10p12 Initialization as Visible Side Effect

The intent of this paragraph is that initialization be considered a
separate access, but this is not explicitly stated.
There is some debate as to whether this needs to be explicitly stated.
In absence of consensus, let those who read the words of this paragraph
apply appropriate common sense.

Core Non-Issue 4: 1.10p13 Initialization as Visible Side Effect

As with 1.10p12, the intent of this paragraph is that initialization
be considered a separate access, but this is not explicitly stated.
There is some debate as to whether this needs to be explicitly stated.
In absence of consensus, let those who read the words of this paragraph
apply appropriate common sense.

WG21 C++-Language Wording

This section lists WG21 C++-language wording.
The corresponding WG14 C-language wording is shown in a separate section
to ease coordination of changes to the two working drafts.

Wording for Core Issue 1

Reword the 1.10p2 as follows:

The value of an object visible to a thread T
at a particular point might beis the
initial value of the object, a value assigned to the object by
T , or a value assigned to the object by another
thread, according to the rules below. [ Note: In some
cases, there may instead be undefined behavior. Much of this
section is motivated by the desire to support atomic operations
with explicit and detailed visibility constraints. However,
it also implicitly supports a simpler view for more restricted
programs. — end note ]

Wording for Core Issue 2

Reword the non–normative note in 1.10p14 to include sequentially consistent
atomic operations as well as lock–based critical sections, as follows:

The execution of a program contains a data race if it contains
two conflicting actions in different threads, at least one of
which is not atomic, and neither happens before the other. Any
such data race results in undefined behavior. [ Note: It can be
shown that programs that correctly use simple locks
and memory_order_seq_cst operations
to prevent all
data races and
that
use no other synchronization operations behave as
the executions of
if the operations executed by
their constituent threads
were
are
simply
interleaved, with each
observed value
value computation
of an object being the
last value assigned
last side effect on that object
in that interleaving. This is normally
referred to as “sequential consistency”. However,
this applies only to
data–race–free programs, and
data–race–free programs
cannot observe most program transformations that do not change
single–threaded program semantics.
In fact, most single–threaded
program transformations continue to be allowed, since any program
that behaves differently as a result must perform an undefined
operation. — end note ]

WG14 C–Language Wording

Change WG14 5.1.2.4p2 as follows:

The value of an object visible to a thread T
at a particular point might beis the
initial value of the object, a value assigned to the object
by T , or a value assigned to the object by another
thread, according to the rules below.

Change WG14 5.1.2.4p23 as follows:

NOTE 11 It can be shown that programs that correctly use simple
locks and memory_order_seq_cst operations
to prevent all data races,that and
use no other synchronization operations, behave
as though the executions ofthe operations
executed by their constituent threads wereare simply interleaved, with each observed
valuevalue computation of an object being
the last value assignedlast side effect on
that object in that interleaving. This is normally
referred to as “sequential consistency”. However,
this applies only to data-race-free programs, and
data-race-free programs cannot observe most program
transformations that do not change single-threaded program
semantics. In fact, most single-threaded program transformations
continue to be allowed, since any program that behaves differently
as a result must contain undefined behavior.